An efficient high speed signaling system requires the use of well-controlled waveforms. For example, in a high speed signaling system with a clock cycle time in the range of approximately one to two nanoseconds, the amplitude of the voltage swing, the rise and fall times, and the duty cycle of the signaling waveform should be within well-defined limits. The term “voltage swing” refers to a difference between a minimum predetermined voltage and a maximum predetermined voltage of a signal. For example, typical limits may include a voltage swing of approximately one volt, a near fifty percent duty cycle, and a rise and a fall time of approximately one hundred picoseconds (ps). In some systems, the voltage swing of CMOS signals ranges from a low of zero volts to a high of five volts. In other systems, the voltage swing of the CMOS signals ranges from a low of zero volts to a high of 2.5 volts.
A receiver system that receives and converts the high-speed, low swing waveforms to CMOS signals requires careful design, especially when multiple high-speed waveforms are received simultaneously and where noise is a significant factor.
The following naming convention will be used for signals. For example, the name “system clock” will refer to one signal, while that signal's complement will be referred to as “system clock B” or “system clock_b.” In other words, the complement of a signal will have an upper or lower case “b” following its name.
As shown in FIG. 1, a prior art sampling receiver 100 has a sense-amplifier 102 and a latch 104. The sense amplifier 102 receives, senses and amplifies small changes in the input signal, Data In, with respect to a reference voltage Vref, and outputs a differential signal, A and A_b. The latch 104 amplifies, stores and converts the differential signal, A and A_b, to predetermined low and high values.
Referring also to the timing diagram of FIG. 2, a system clock and its complement, system clock_b, control the operation of the sampling receiver 100. When system clock_b is transitions low the sense amplifier 102 is disabled. Two linear load/precharge transistors 112, 114 become active and pull signals A and A_b at nodes NA and NA_b to a high voltage level.
When system clock_b transitions high, the sense amplifier 102 is enabled and senses the voltage of the data input signal, Data In. The two linear load transistors 112, 114 become inactive. When the voltage of the data input signal, Data In, at the gate of input transistor 116 exceeds the reference voltage VREF at the gate of transistor 118, the input transistor 116 becomes active and pulls output signal A_b to a low voltage level via a current sink 120. When the data input signal is less than or equal to the reference voltage VREF, the input transistor 116 is inactive (i.e., or at least less conductive than transistor 118) and the output signal A_b remains high.
The cross-coupled transistor pair 122, 124 stores the state of signals A and A_b. Initially, when system clock_b is low, transistors 112 and 114 are enabled and act as linear load devices to the differential pair 116, 118. When system clock_b transitions high, transistors 112 and 114 become inactive and the cross-coupled pair 122, 124 is enabled to sense and amplify variations of the input data signal, Data In. When the voltage of the input signal, Data In, is less than the reference voltage VREF, transistor 118 is active and pulls the voltage of signal A at node NA to ground, which causes the voltage of signal A_b at node NA_b to transition high. When the voltage of the input signal Data In exceeds the reference voltage VREF, transistor 116 becomes active and pulls the signal A_b at node NA_b low; in addition, transistor 118 becomes inactive and the signal A_b at node NA_b is pulled high. The cross-coupled pair 122, 124 acts as an amplifier for small changes in the voltage of the input signal Data In with respect to the reference voltage VREF.
When system clock_b transitions low, sense amplifier 102 is disabled and the complementary output signals A and A_b from the sense amplifier 102 are stored in latch 104. Latch 104 is enabled by the system clock.
In latch 104, an equalizing transistor 126 becomes active when the system clock transitions low and drives the output signals Out and Out_b to the same voltage level. When the system clock transitions high, the equalizing transistor 126 becomes inactive, latch-enable transistors 128, 130 become active and enable the latch-data-input transistors 132, 134 to act as a pull-down circuit when responding to the differential output signals A and A_b from the sense amplifier 102. In particular, when the system clock is high, the latch-data-input transistors 132, 134 are responsive to the amplified signals A and A_b. A four transistor latch circuit 136 latches the associated state of signals A and A_b, and generates the latched-output signals, Out 142 and Out_b 144. The four transistor latch circuit 136 includes transistors 152, 154, 156 and 158.
When the system clock transitions low, latch-enable transistors 128, 130 become inactive thereby causing the latch 104 to become non-responsive to signals A and A_b. In this way, the latch 104 captures the state of A and A_b at the high-to-low transition of the system clock. To reduce the probability of errors caused by noise, the timing of the high-to-low transition of the system clock should occur at a time when the differential amplitude between the A and A_b signals is greatest. In addition, when a system has multiple receivers and drivers operating simultaneously, the likelihood of noise injection on VREF and, consequently, errors is increased.
The circuit of FIG. 2 senses even data values, D0 and D2, in response to the falling edge of the system clock, and latches the even data values in response to the rising edge of the system clock. Another circuit, similar to the circuit of FIG. 2, operates on opposite clock edges senses and latches odd data values (e.g., sensing in response to the rising edge of the system clock) and latching in response to the falling edge of the system clock.